The present invention relates generally to digital timing circuits and, more particularly, to a digital time fuze. In the past, it was usual for electronic time fuzes to employ an RC timing circuit. These were normally energized from the battery which required considerable space, weight, and cost, and had a finite shelf life as well. Further, the RC components had to be precise and were required to maintain the values unchanged for considerable cperiods of time. Additionally, elaborate temperature compensation was necessary in order to produce accurate results. Such RC timing circuit requirements presented serious problems in the design of electronic time fuzes of the past.
More recently, an analog time fuze was proposed in U.S. Pat. No. 3,502,024 to Mountjoy. The time fuze of the Mountjoy patent is a battery-less fuze which has a large capacitor to supply the energy for both the timing circuit and the operation of the detonator at the conclusion of the timing cycle. This fuze employs a timing circuit which is relatively insensitive to the capacitance of the large capacitor and permits compensation for variations from standard values of the capacitive and resistive components used within the timing circuit. Such compensation takes the form of voltages supplied just prior to launching of the missile or projectile.
The Mountjoy patent further discloses a means for charging the energy supply capacitor in the fuze, as well as supplying information on the desired run time of the fuze, through use of a single wire connection and voltages of opposite polarities. By run time, it is meant the time period between launching of the missile or projectile and the firing of the detonator.
Despite the advances represented by the Mountjoy patent disclosure, the time fuze disclosed therein has an inherent run time accuracy determined by the precision of certain of its components. This timing accuracy generally decreases with increasing run time of the fuze. Additionally, for longer run times it is conceivable that larger values of the RC timing components would be required in order to obtain acceptable accuracy for the period.
Moreover, the accuracy of the analog fuze is obtained by way of either explicit knowledge of component values of the resistive and capacitive elements in the fuze or by some measurement or compensation scheme of the time-setting mechanism circuitry. Such compensation techniques require precise measurement and complex setting circuitry to calculate and set the run time of the fuze. Even so, other effects such as temperature drifts cannot be easily compensated for.
Another major contributor toward analog fuze inaccuracy, which cannot easily be compensated for, is a capacitor defect known as dielectric absorption. This effect is very pronounced for longer fuze run times with high value capacitors. Dielectric absorption effects are not obvious from a direct capacitance measurement. It cannot be easily compensated for by the conventional setters. By setter, it is meant external equipment which supplies run time information to the fuze, such run time information being calculated within the setter according to known parameters of the time fuze and externally dictated time of flight requirements.
Another disadvantage of the Mountjoy fuze involves the bipolar nature of the charging signature, i.e., the signal by which the energy for running the circuit is supplied to the energy storage capacitor, as well as the run time information supplied to the timing circuit itself. This bipolar signal precludes easy signal multiplexing which would allow both fuze charging and setting, and rocket firing from the same signal wire.